Memory cell dies are formed by patterning a wafer. In some instances, a single patterned wafer contains thousands of memory cell dies. During a manufacturing process, variations occur between different memory cell dies on the wafer, as well as between different wafers. While many of the memory cell dies pass quality control standards, some of the memory cell dies exhibit an unacceptable functionality during read or write operations.
Unacceptable functionality during a read operation is termed read disturb. Read disturb is a result of a change in a threshold voltage of transistors in a memory cell die which in some instances causes the read operation to produce a value opposite to the actual stored value. In order to reduce read disturb to an acceptable level, a word line of the memory cell die is underdriven to a value below an operating voltage. This underdriving is called read assist. However, cells equipped with read assist exhibit a reduced switching speed. In some instances, the switching speed is reduced by about 27%.
In addition to read assist, some memory cell dies are enabled with write assist functionality, in order to pass quality control tests. Write assist is achieved by providing a negative charge on a bit line. The negative charge on the bit line allows the memory cell die to function at a lower operating voltage. However, the write assist functionality increases power consumption of the memory cell die because a voltage supply is used to negatively charge the bit line.
Memory cell die manufacturers either accept a lower production yield or enable every memory cell die on the wafer with read assist or write assist functionality. Enabling every memory cell die with read assist functionality reduces the switching speed of all the memory cell dies, even those having acceptable read operations. Enabling every memory cell die with write assist functionality increases the power consumption of all the memory cell dies, even those having acceptable write operations.